Dynamics of Lake Michigan Phytoplankton: Recent Changes in Surface and Deep Communities

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Abstract
Lake Michigan phytoplankton dynamics were studied from the end of spring isothermal mixing (May) through midstratification (July–August) in 1982–84. Phytoplankton composition shifted from a diatom-dominated community (75% of phytoplankton carbon) during May to a phytoflagellate-dominated community (71% of phytoplankton carbon) during July–August. This summer phytoflagellate dominance in the 1980s is distinctly different from the summer blue-green and green algae dominance in the 1970s. Foodweb interactions caused by a changing zooplankton composition and nutrient supply changes were two possible causes. A deep chlorophyll layer (DCL) developed after the onset of thermal stratification. The DCL initially developed in the 15–30 m region and deepended to 25–50 m in July and the 40–70 m region in August. The DCL in 1982–84 was larger in size and located deeper in the water column than those reported from the 1970s. This difference was related to increases in light transparency found in the 1980s that were a result of increased zooplankton grazing pressure.